Overload protection means of the electrothermal type have been known heretofore. For example, U.S. Pat. No. 4,486,733, P. K. Moldovan et al, dated Dec. 4, 1984, shows an electrothermal responsive protective mechanism of the bimetal type having also an ambient temperature compensating bimetal element to maintain accurate operation of the protective mechanism despite ambient temperature changes. In the mechanism shown in that patent, the overload tripping force is provided by the relatively cooler ambient temperature compensating bimetal element rather than by the hot electrically heated bimetal element to prevent overstress of the latter. It will be apparent that the thermal mechanism shown in that patent is not a self contained unit or subassembly but instead all of the components thereof must be individually assembled into the housing of the electrical device such as a circuit breaker/switch with which it is used. While thermal mechanisms of the type shown in that patent have been useful for their intended purposes, it has been found desirable to provide improvements thereover while at the same time retaining the good features thereof.